SA imaging can be used to increase resolution beyond the diffraction limit of a physical aperture of an imaging system. In SA imaging systems, a large “virtual” aperture is synthesized along a path by coherently summing the amplitude and phase information of return echoes from a plurality of electromagnetic signals sequentially transmitted by a relatively small physical aperture provided on a platform moving along the path. SA imaging systems generally include a transmitter-receiver unit mounted on an airborne, spaceborne, or terrestrial platform traveling along a path over a target region to be imaged. The transmitter-receiver unit directs a plurality of electromagnetic signals onto the target region and collects a series of phase-coherent return echoes corresponding to the electromagnetic signals reflected by the target region. The return echoes can be recorded, and then coherently combined using signal processing techniques to reconstruct a high-resolution image of the target region.
SA imaging was initially developed and has been successfully employed at radio frequencies, where it is referred to as “synthetic aperture radar” (SAR) imaging. Conventional SAR systems typically operate in the centimeter (cm) wavelength range and produce images with azimuth resolutions of the order of a decimeter (dm) to a meter (m), depending on the applications. As resolution is generally inversely proportional to the wavelength used for imaging, there has been a growing interest to extend SAR technology to shorter wavelengths. In this context, an emerging technology referred to as “synthetic aperture ladar” (SAL) imaging has been developed to apply SAR technology to the visible and near-infrared portions of the electromagnetic spectrum. It is envisioned that SAL could produce images with azimuth resolutions of centimeters or less, and provide information complementary to that provided by SAR systems. Most implementations of SAL imaging are based on coherent detection with chirped signals. In coherent detection, the return signal reflected by the target is mixed with an LO reference signal. The mixing of the return signal with the LO signal results in the generation of a beat signal having a frequency equal to the difference between the frequencies of the two mixed signals. The beat frequency depends on the difference between the path length of the LO signal and the path length of the main signal from the source to the target and back to the detector.
A challenge in SAL imaging lies in the adjustment of the optical path length of the LO signal to match the round-trip path length of the transmitted/returned (main) signal, to ensure that the spectrum of the beat signal falls within the bandwidth of the detector. One existing approach devised to tackle with this challenge is to use an optical delay, for example an optical fiber, to delay the LO signal by an amount that is approximately equal to the round-trip time to the target. A drawback of this approach is that different delay lines must be used for different target ranges, thus preventing or hindering the ability to make real-time or near real-time adjustment of the relative path length difference between the main and LO signals. Another challenge in SAL imaging is the measurement and correction of phase errors. As SAL imaging relies on coherent detection, it is susceptible to laser phase noise. Laser phase noise arises from the finite coherence length and other instabilities of laser sources and causes phase errors that can degrade the image reconstruction process. Furthermore, any uncompensated fluctuations in the relative path length difference, or relative temporal delay, between the main signal and the LO signal can affect the phase of the measured signal and, in turn, lead to phase errors that impair the integrity of the measured signals. Challenges therefore remain in the field of SAL imaging involving LO delay adjustment and associated phase-error compensation.